When a user of a medical syringe administers a substance, it is common, in particular within certain areas of treatment, to distribute the total volume carried by the syringe over an area. Some examples of such areas of treatment are dermatology, plastic surgery, cosmetic surgery, and odontology. The distribution is for instance done by inserting the needle of the syringe just beneath and approximately in parallel with the skin surface and then administering the substance while withdrawing the needle. Another way is to administer the substance at several anatomic locations within a limited area, i.e. to administer a fraction of the total volume at each location. In both these, and other, methods of distributing the substance it is an advantage if the syringe is provided with some kind of dosing aid, which aids the user in knowing how large fraction of the total volume of the substance is administered at each location or per time unit. Conventionally, the syringe barrel is provided with a gradation scale, and the user observes the position of the plunger along the scale. However, the scale is difficult to see and might be partly or fully covered by a sticker, or the like, and the syringe is at times held at an angle where the scale is hidden. Further, a main problem with the use of a gradation scale as such is the fact that a user has to observe it during injection which means that a distraction of attention arises when the user has to look at the gradation scale every now and then instead of focusing on the course of events at the treatment area. Mechanic or electronic injectors providing an accurate dosing have been used, but they are more expensive and heavier than syringes and many users prefer to hold the injection device in the way that a conventional syringe is held. Further, most injectors do not allow a user to perform needle aspiration prior to injection of the medicament. This is a serious drawback since it does not allow a user to realize that injection has been done in a blood vessel.
An effort to provide a syringe with a dosing aid is disclosed in WO 2008/057976, and in WO 2008/016381, where an engagement member is provided on the finger grip is movable between an inactive, or non-engaging, position and an active position where it engages with a structure of the plunger rod. However, in WO 2008/057976 it is unclear how the movement is actually realized due to a most schematic illustration and description thereof. In WO 2008/016381 the operation of moving the engagement member between the two positions is uncomfortable due to the structure providing for that operation.